1. Field of the Invention
The present invention relates to a refrigerating oil composition and more particularly, to a refrigerating oil composition which includes a refrigerant containing a hydrocarbon compound as a predominant component.
2. Background Art
In general, compression-type refrigerators includes a compressor, a condenser, an expansion valve, and an evaporator and have a structure in which a liquid mixture of a refrigerant and a lubricating oil is circulated in a closed system. In such compression-type refrigerators, generally, the inside temperature of the compressor rises to 50° C. or higher, and that of the condenser is at about −40° C., although these temperatures may vary depending on the model of the refrigerators. Therefore, a refrigerant and a lubricating oil are typically required to impart lubricity to movable members of the refrigerator and to be circulated in the refrigerator such that at least portions of the two liquids are in a mutually miscible state within a temperature range of −40° C. to +50° C.
Heretofore, chlorofluorocarbons such as dichlorofluoromethane (R-12) and chlorodifluoromethane (R-22) have been generally employed as a refrigerant for the compression-type refrigerators, and various types of mineral oils and synthetic oils have been employed as lubricating oils. However, since chlorofluorocarbons such as R-12 and R-22 may cause environmental pollution issue due to, for example, such as ozonosphere depletion, hydrogen-containing chlorofluoro compounds which are less destructive to ozonosphere, inter alia, 1,1,1,2-tetrafluoroethane (R-134a), has attracted attention in recent years.
However, these hydrogen-containing chlorofluoro compounds might aggravate global warming and, therefore, use of natural-substance-based cooling media that do not raise the above problems have been reevaluated. Among such cooling media, a hydrocarbon-based refrigerant is now under investigation.
In the case where a hydrocarbon-based refrigerant is used in a compression-type refrigerator, when a conventionally employed mineral oil or alkylbenzene is used as a lubricating oil, the base oil and the refrigerant are completely dissolved each other, thereby reducing viscosity of the base oil; i.e., reducing lubrication performance. As a result, wear resistance of members of the refrigerator and sealing performance are reduced, thereby causing, for example, difficulty in reliable operation for a long period of time.
Since the hydrocarbon-based refrigerant has flammability, reduction of the amount of the refrigerant added to a refrigerating system is investigated from the viewpoint of lowering influence of leakage thereof to the outside. The refrigerant dissolved in refrigerating oil provides decreased cooling effect. Thus, the refrigerant must be incorporated in a considerably large amount so as to compensate the portion of the refrigerant dissolved in lubricating oil. Therefore, reduction of such a large amount of refrigerant is a critical issue.
As described above, when a hydrocarbon-based refrigerant is used, a lubricating oil is required to have miscibility with respect to the refrigerant at such a degree that the lubricating oil can be circulated in the refrigerator and impart lubricity to movable members of the refrigerator. On the other hand, from the viewpoint of reduction of amount of a hydrocarbon-based refrigerant employed, the miscibility of the refrigerant is desired to be as small as possible.
In an attempt to solve the aforementioned problems, a variety of refrigerating oils including a hydrocarbon-based refrigerant have been proposed. For example, Japanese Patent Application Laid-Open (kokai) No. 11-349970 (in claims) discloses a refrigerating oil containing at least one member selected from a polyethylene glycol-based oil, a polypropylene glycol-based oil, a polyethylene glycol-polypropylene glycol copolymer-based oil, and polyvinyl ether-based oil.
Japanese Patent Application Laid-Open (kokai) No. 11-349970 discloses the aforementioned polyalkylene glycol oil and other materials in terms of molecular weight and end structures effective for use in combination of a hydrocarbon-based refrigerant. However, the patent document does not disclose mutual solubility of the hydrocarbon-based refrigerant and the lubricating oil in actual use in combination, mixture viscosity which serves as an index for the performance of the lubricating oil in which the refrigerant has been dissolved, and other properties. Therefore, the scope of the effective species among the disclosed polyalkylene glycol oil and other materials has not yet been confirmed.
Regarding polyalkylene glycol oil, the present applicant previously studied the relationship between the end structure of a polyethylene glycol (EO)-polypropylene glycol (PO) copolymer-based oil and the EO unit/PO unit ratio, and filed an application (see Japanese Patent Application Laid-Open (kokai) No. 10-158671, claims).